The present invention relates to a liquid crystal display device, in particular, to a plasma addressed liquid crystal display device of a flat panel structure including a liquid crystal cell and a plasma cell arranged in layers.
Development of plasma addressed liquid crystal display devices (hereinafter, abbreviated as PALCD devices) is in progress for realization of large and thin flat displays. For example, a PALCD device is disclosed in Japanese Laid-Open Patent Publication No. 1-217396.
FIG. 20 schematically illustrates a conventional PALCD device 200. The PALCD device 200 has a layered structure composed of a liquid crystal cell 201 and a plasma cell 202 with a dielectric sheet 203 interposed therebetween. A pair of polarizing plates 213 and 214 sandwich the liquid crystal cell 201 and the plasma cell 202. Typically, a backlight (not shown) is disposed on the back of the plasma cell 202.
The plasma cell 202 includes: an insulating substrate 204 having a plurality of parallel stripe grooves 205 formed therein; and the dielectric sheet 203 that serves as part of the liquid crystal cell 201 as will be described later. Each of the plurality of grooves 205 formed in the substrate 204 is sealed with the dielectric sheet 203. The sealed space of the groove is filled with gas ionizable with discharge, forming a plasma channel 205 (denoted by the same reference numeral as the groove). A pair of plasma electrodes 206 and 207 are placed on the bottom of each groove 205. A voltage is applied to the confined gas through the plasma electrodes that serve as an anode A and a cathode K, to ionize the gas and thus generate plasma discharge. The ionization of the gas in the plasma channel 205 under the plasma discharge is also called xe2x80x9cactivationxe2x80x9d of the plasma channel 205 in some cases.
The liquid crystal cell 201 includes a substrate 208, the dielectric sheet 203, and a liquid crystal layer 209 interposed between the substrate 208 and the dielectric sheet 203. A plurality of parallel stripe electrodes (column electrodes) 210 are formed on the surface of the substrate 208 facing the liquid crystal layer 209. The electrodes 210 extend to cross the plasma channels 205. Also formed on the surface of the substrate 208 facing the liquid crystal layer 209 are colored layers (not shown) formed at positions corresponding to the respective electrodes 210 and a black matrix 212 filling spaces between the colored layers. The colored layers typically include red, green, and blue layers (see FIG. 22A).
Pixel regions are formed in the respective crossings of the electrodes 210 and the plasma channels 205. The portion of the liquid crystal layer 209 located in each of the pixel regions changes its orientation state depending on the voltage applied between the electrode 210 and the plasma channel 205, thereby changing the amount of light passing through the pixel region. By applying video signals to the portions of the liquid crystal layer 209 located in the respective pixel regions arranged in a matrix as a whole, the amounts of light passing through the respective pixel regions are controlled, whereby an image is displayed. As used herein, the minimum display unit is referred to as a xe2x80x9cpixelxe2x80x9d, and the region of an LCD device that corresponds to each xe2x80x9cpixelxe2x80x9d is referred to as a xe2x80x9cpixel regionxe2x80x9d. Each pixel region exists in each crossing of the plasma channel 205 and the electrode 210. In a typical conventional LCD device having a black matrix, each pixel region exists in each opening of the black matrix. In other words, the opening of the black matrix defines the outline of the pixel region. In principle, pixel regions as well as pixels do not overlap each other. However, as will be described later in detail, if a crosstalk phenomenon occurs in a PALCD device, a xe2x80x9cpixelxe2x80x9d and a xe2x80x9cpixel regionxe2x80x9d as defined in actual display operation overlap at least part of the adjacent xe2x80x9cpixelxe2x80x9d and xe2x80x9cpixel regionxe2x80x9d, respectively. That is, the xe2x80x9cpixel regionxe2x80x9d in actual display operation does not match the xe2x80x9cpixel regionxe2x80x9d in design (structure). Herein, the xe2x80x9cpixel region in displayxe2x80x9d and the xe2x80x9cpixel region in designxe2x80x9d are often used to distinguish one from the other.
In the conventional PALCD device 200, the plasma channels 205 serve as row scanning units while the electrodes 210 serve as column scanning units, for example. Linear sequential scanning is carried out by activating the plasma channels 205 selectively in succession. In synchronization with this scanning, a video signal is applied to the electrodes 210 constituting the column drive units. The selectively activated plasma channel 205, which is filled with ionized gas, is entirely turned to an anode potential (also called a xe2x80x9creference potentialxe2x80x9d). In this state, when a drive voltage (corresponding to a video signal voltage) is applied between the plasma channel 205 and the electrode 210 facing each other via the dielectric sheet 203 and the liquid crystal layer 209, charges of the amount corresponding to the potential difference between the anode potential and the drive potential are induced to and accumulated on the bottom surface 203S of the dielectric sheet 203 (the surface facing the plasma channel 205, which is hereinafter called a xe2x80x9cdielectric bottom surface 203S). Next, when this plasma channel 205 is made non-selected (plasma discharge is stopped), the plasma channel 205 is put in an insulated state. Thus, the charges are kept accumulated on the dielectric bottom surface 203S until the plasma channel 205 is selected and activated next time. As a result, the potential difference (voltage) between the dielectric bottom surface 203S and the electrode 210 is maintained. In other words, the voltage corresponding to the drive voltage that had been applied to the corresponding electrode 210 when the plasma channel 205 was selected (the drive voltage itself if the anode voltage was the ground voltage) is sample-held by the existence of capacitances formed by the dielectric bottom surface 203S/dielectric sheet 203/liquid crystal layer 209/electrode 210. In this way, the plasma channel 205 functions as a switching element that controls electrical connection/disconnection between the dielectric bottom surface 203S and the anode electrode 207. The dielectric bottom surface 203S serves as a virtual electrode. The rows and the columns may be reversed so that the drive voltage is applied to the anode electrodes 207 of the plasma channels 205 while the scanning voltage is applied to the electrodes 210.
The pixel region of the PALCD device 200 can be represented by an equivalent circuit shown in FIG. 21. Referring to FIG. 21, one pixel region of the PALCD device 200 is essentially composed of: a capacitance CG (dielectric sheet capacitance) including the dielectric bottom surface 203S and the dielectric sheet 203; a capacitance CLC (liquid crystal capacitance) including the liquid crystal layer 209 serially connected to the capacitance CG; and the anode electrode 207 connected to the dielectric bottom surface 203S via a switch S (plasma channel 205). A drive voltage VD is externally applied to the electrode 210. When the switch S is turned ON, the drive voltage VD (AC voltage; VD is absolute) is applied between the dielectric bottom surface 203S and the electrode 210. At this time, it is a voltage VLC applied to the liquid crystal capacitance CLC that directly influences the display state of the pixel region (the orientation state of liquid crystal molecules in the liquid crystal layer 209). The voltage VLC is given by expression (1) below.
VLC=VDxc3x97{CG/(CLC+CG)}xe2x80x83xe2x80x83(1)
In other words, the drive voltage is divided between the serially connected capacitance CLC and capacitance CG. Assuming that the thickness of the liquid crystal layer 209 is dLC, that of the dielectric sheet 203 is dG, and the relative dielectric constants of the liquid crystal layer 209 and the dielectric sheet 203 (typically, glass) are equal to each other, VLC is given by expression (2) below.
VLC=VDxc3x97{dLC/(dLC+dG)}xe2x80x83xe2x80x83(2)
Typically in the PALCD device 200, the thickness dLC of the liquid crystal layer 209 is about 5 xcexcm, and the thickness dG of the dielectric sheet 203 (glass sheet) is about 50 xcexcm. From expression (2), therefore, it is found that in order to apply a voltage of 4 V to the liquid crystal layer 209 (VLC=4 V), for example, a voltage of about 40 V needs to be applied as the drive voltage VD.
The PALCD device 200 is especially an expected candidate of a large-size LCD device for the reason that it can be fabricated in a simple process compared with LCD devices using thin film transistors, among others.
Meanwhile, for improvement of the viewing angle characteristics of twisted nematic (TN) mode LCD devices, the present inventors disclosed axially symmetrically aligned microcell (ASM) mode LCD devices in Japanese Laid-Open Patent Publication Nos. 6-301015 and 7-120728, for example. The liquid crystal layer of an ASM mode LCD device is divided into a plurality of liquid crystal regions by polymer walls. Liquid crystal molecules in each of the liquid crystal regions are aligned axially symmetrically with respect to an axis (a symmetry axis) vertical to the display plane (the surface of a substrate constituting the LCD device). The liquid crystal regions are typically formed every pixel region. With the axially symmetrically aligned liquid crystal molecules, the ASM mode LCD device exhibits display that is small in contrast change whichever direction the display is viewed. That is, the ASM mode LCD device has wide viewing angle characteristics.
The polymer walls for aligning liquid crystal molecules axially symmetrically are formed by polymerization-inducing phase separation of a mixture of a polymerizable material and a liquid crystal material (see the above publications). Alternatively, polymer walls may be formed on a substrate in advance by a photolithography process using a photosensitive resin (see Japanese Laid-Open Patent Publication No. 10-186330, for example).
The present inventors further disclosed a PALCD device adopting the ASM mode in Japanese Laid-Open Patent Publication No. 11-167099, for example. Such an ASM mode PALCD device is expected to be a very promising large screen display having wide viewing angle characteristics.
The above conventional devices have problems as follows. The PALCD device 200 has a problem of easily generating a crosstalk phenomenon. To suppress/prevent the crosstalk phenomenon, the present inventors investigated the cause of occurrence of the crosstalk phenomenon in detail and found the following.
In the PALCD device, a crosstalk phenomenon presumably occurs because a large drive voltage is applied to the dielectric sheet and the liquid crystal layer as described above. Referring to FIGS. 22A and 22B, the crosstalk phenomenon in the PALCD device 200 will be described.
FIGS. 22A and 22B are a schematic cross-sectional view and a top view, respectively, of the PALCD device 200, specifically illustrating three continuous pixel regions P along one plasma channel 205. The pixel regions P herein indicate pixel regions in design. A PALCD has periodic structures in the row and column directions. Accordingly, illustration of structures, which have functionally nothing to do with the pixel region shown at the center, may be omitted, as in FIGS. 22A and 22B.
The liquid crystal layer 209 of the PALCD device 200 includes liquid crystal molecules 209a having negative dielectric anisotropy. The pair of polarizing plates 213 and 214 are disposed in the crossed-Nicols state. The PALCD device 200 performs display in the normally black mode. Red, green, and blue layers are used as the colored layers (not shown) formed on the surface of the substrate 208 facing the liquid crystal layer 209, and the three pixel regions P correspond to red (R), green (G), and blue (B).
The pixel region P (in design) of the PALCD device 200 is defined by the following. The width of the pixel region P along the length of the plasma channel 205 (orthogonal to the length of the electrode 210) is defined by the opening of the black matrix 212, which is equal to the width WEL of the electrode 210 in the illustrated example. The width of the pixel region P along the length of the electrode 210 (orthogonal to the length of the plasma channel 205) is defined by the width WPC of the plasma channel 205 (distance between the side ribs or between the electrodes for plasma generation).
FIGS. 22A and 22B illustrate the state where, after a drive voltage (a voltage equal to or more than a threshold voltage of the liquid crystal layer; 40 V, for example) was applied to an electrode 210G corresponding to the center pixel electrode P while the plasma channel 205 was being activated, the applied voltage is retained in the center pixel electrode P (in this state, the plasma channel 205 is already in the insulated state). That is, the center pixel electrode P (green) is on the ON state, while the two adjacent pixel electrodes P (red and blue) are on the OFF state.
If a crosstalk phenomenon occurs, the PALCD display device 200 is observed as shown in FIG. 22B. That is, the hatched black matrix 212 portions and cross-hatched areas in FIG. 22B are observed black (dark). Originally, only the center pixel region p (green) should be in the ON (bright) state, and the adjacent pixel regions P (red and blue) should be in the OFF (dark) state. Actually, however, part of the pixel regions P corresponding to red and blue colors adjacent to the ON-state pixel region P (green) are observed as the ON state. To state differently, the width of the portion of the liquid crystal layer 209 turned ON along the length of the plasma channel 205 (that is, the width of the pixel region in display) is larger than the width of the pixel region P. In other words, the pixel region in display overlaps part of the adjacent pixel regions. Thus, if a crosstalk phenomenon occurs, color purity degrades (color becomes whitish) in color display because green to be originally displayed is mixed with red and blue. In monochrome display, the contour is blurred.
FIGS. 23A and 23B are a schematic cross-sectional view and a top view, respectively, of a PALCD device 1700, specifically illustrating three continuous pixel regions P along one plasma channel 1705. The pixel regions P herein indicate pixel regions in design.
The PALCD device 1700 includes a liquid crystal layer 1709 divided into a plurality of liquid crystal regions 1709a by dielectric structures (polymer walls) 1720a, 1720b, 1720c, and 1720d. A pair of polarizing plates 1713 and 1714 are disposed in the crossed-Nicols state. The liquid crystal layer 1709 includes liquid crystal molecules 1709b having negative dielectric anisotropy. Vertical alignment films (not shown) are formed in contact with the top and bottom surfaces of the liquid crystal layer 1709. The liquid crystal molecules 1709b in the respective liquid crystal regions 1709a are aligned axially symmetrically with respect to an axis SA vertical to the surface of a substrate 1708. The PALCD device 1700 performs display in the normally black mode. Red, green, and blue colored layers (not shown) are formed on the surface of the substrate 1708 facing the liquid crystal layer 1709, and the three pixel regions P correspond to red (R), green (G), and blue (B). A black matrix 1712 formed between the colored layers generally has a width larger than the width of the dielectric structures 1720a formed between the adjacent electrodes 1710. This is for the reason of keeping the display quality from degrading even if the dielectric structures 1720a are displaced due to misalignment during the formation of the dielectric structures 1720a. 
Each of the pixel regions P (in design) of the PALCD device 1700 is defined by the following. The width of the pixel region P along the length of the plasma channel 1705 (orthogonal to the length of the electrode 1710) is defined by the opening of the black matrix 1712, which is equal to the width WEL of the electrode 1710 in the illustrated example. The width of the pixel region P along the length of the electrode 1710 (orthogonal to the length of the plasma channel 1705) is defined by the width WPC of the plasma channel 1705 (distance between the side ribs or between the electrodes for plasma generation). As described above, the width of the dielectric structures 1720a formed between the adjacent electrodes 1710 is smaller than the width of the black matrix 1712. Therefore, the dielectric structures 1720a will not exist in portions of the pixel region P including the sides (periphery sides) thereof orthogonal to the length of the plasma channel 1705.
FIGS. 23A and 23B illustrate the state where, after a drive voltage (a voltage equal to or more than a threshold voltage of the liquid crystal layer; 40 V, for example) was applied to an electrode 1710G corresponding to the center pixel electrode P while the plasma channel 1705 was being activated, the applied voltage is retained in the center pixel electrode P (in this state, the plasma channel 1705 is already in the insulated state). That is, the center pixel electrode P (green) is on the ON state, while the two adjacent pixel electrodes P (red and blue) are on the OFF state.
If a crosstalk phenomenon occurs, the PALCD display device 1700 is displayed as shown in FIG. 23B. That is, the hatched black matrix 1712 portions and cross-hatched areas in FIG. 23B are observed black (dark). Originally, only the center pixel region p (green) should be in the ON (bright) state, and the adjacent pixel regions P (red and blue) should be in the OFF (dark) state. Actually, however, part of the pixel regions P corresponding to red and blue colors adjacent to the ON-state pixel region P (green) are observed as the ON state. To state differently, the width of the portion of the liquid crystal layer 1709 turned ON along the length of the plasma channel 1705 (that is, the width of the pixel region in display) is larger than the width of the pixel region P. In other words, the pixel region in display overlaps part of the adjacent pixel regions. Thus, if a crosstalk phenomenon occurs, color purity degrades (color becomes whitish) in color display because green to be originally displayed is mixed with red and blue. In monochrome display, the contour is blurred.
The state of the PALCD device 200 during occurrence of a crosstalk phenomenon will be described in detail with reference to FIG. 22A.
When a drive voltage (40 V, for example) is applied to the center electrode 210G while the plasma channel 205 is being activated (meanwhile, 0 V is applied to adjacent electrodes 210R and 210B), charges are induced to and accumulated on the portion of the bottom surface 203S of the dielectric sheet 203 facing the electrode 210G. The amount of charges is determined depending on the magnitude of the drive voltage and the value of capacitance (value of serially connected CG and CLC). An electric field (electric lines of power) generated by the drive voltage applied to the electrode 210G has been widened to some extent when it reaches the dielectric bottom surface 203S. Accordingly, the width WG of the portion of the dielectric bottom surface 203S on which charges are accumulated is larger than the width WEL of the electrode 210G. An electric field (electric lines of power) E returned by the accumulated charges toward the electrode 210G is further wider than the width WG of the charge-accumulated portion. As a result, in the adjacent off-state pixel regions, that is, the pixel regions including the electrodes 210R and 210B to which 0 V had been applied when the plasma channel 205 was activated, orientation of the liquid crystal molecules 209a in the liquid crystal layer 209 located near the electrode 210G are influenced by the electric field (voltage) generated by the accumulated charges. Thus, as shown in FIG. 22A, the portion of the liquid crystal layer 209 where the liquid crystal molecules 209a having negative dielectric anisotropy are oriented vertically to the direction of the electric field expands into the adjacent pixel regions.
The crosstalk phenomenon due to a leak electric field (voltage) generated by charges accumulated on the dielectric bottom surface 203S is herein called data diffusion crosstalk (DDC). The leak electric field (voltage) as used herein refers to an electric field (voltage) leaking beyond the ON-state pixel region P. The influence of DDC on the PALCD device 200 was quantitatively evaluated by simulation. The results are shown in FIG. 24.
The x-axis of the graph in FIG. 24 represents the position with respect to the electrodes 210, and the y-axis thereof represents the relative value of the charge distribution on the dielectric bottom surface 203S (solid line) and the electric field distribution in the liquid crystal layer 209 (broken line). In the simulation, a 42-inch VGA-compatible PALCD device was assumed. That is, the width of the electrodes 210 was 324 xcexcm, the distance between the electrodes 210 was 40 xcexcm, the thickness of the liquid crystal layer 209 was 6 xcexcm, and the thickness of the dielectric sheet 203 was 50 xcexcm. The relative dielectric constants of the liquid crystal layer 209 and the dielectric sheet 203 were assumed to be the same. As in the state shown in FIGS. 22A and 22B, a drive voltage (maximum gray scale voltage, specifically 80 V) was applied only to the center electrode 210G.
As is observed from the solid line in FIG. 24, the width of the portion of the dielectric bottom surface 203S on which charges are accumulated (WG in FIGS. 22A and 22B) is larger than the width of the electrode 210G (WEL in FIGS. 22A and 22B). As is observed from the broken line in FIG. 24, the electric field distribution formed in the liquid crystal layer 209 is wider than the charge distribution on the dielectric bottom surface 203S, spreading to as far as almost the centers of the adjacent electrodes 210R and 210B (adjacent pixel regions). It is found that an electric field of roughly 10% of the maximum electric field is generated in the periphery portions of the electrodes 210R and 210B near the electrode 210G.
The above results indicate that the DDC described above is one of major causes of the crosstalk phenomenon in the conventional PALCD device 200. Presumably, this also applies to the crosstalk phenomenon in the PALCD device 1700 shown in FIGS. 23A and 23B.
Another major cause of the crosstalk phenomenon is due to the potential difference between adjacent electrodes. This crosstalk is herein called side to side crosstalk (SSC). Since SSC is caused by the potential difference between adjacent electrodes (that has many variations), it is difficult to quantitatively evaluate how largely SSC influences actual display. However, it is presumed that SSC is also a major cause of the crosstalk phenomenon in the PALCD device in addition to DDC.
Referring to FIG. 25, if a potential difference exists between the adjacent electrodes 210G and 210B of the conventional PALCD device 200 (or 1700), a lateral electric field (electric lines of power) E from the electrode 210G to the electrode 210B is generated. Due to this lateral electric field E, the liquid crystal molecules 209a existing near the electrode 210B that should originally be oriented in the OFF state is turned to the ON state. If SSC is generated as illustrated in the normally black mode LCD device using liquid crystal molecules 209a having negative dielectric anisotropy, the liquid crystal molecules 209a attempt to orient vertically to the lateral electric field. As a result, part (periphery zone) of the adjacent pixel region that should originally be in the black display (OFF) state is turned to the white display (ON) state (the resultant appearance is substantially the same as that shown in FIG. 22B). Since the periphery zones of the adjacent pixel regions that should originally be in the black display state are turned to the white display state, color purity degrades and thus display quality noticeably degrades in color display. In monochrome display, the contour is blurred.
The liquid crystal display device of the first aspect of the present invention includes a substrate; a dielectric layer; a liquid crystal layer interposed between the substrate and the dielectric layer; a plurality of stripe electrodes formed on a surface of the substrate facing the liquid crystal layer, the electrodes running in a first direction; and a plurality of stripe plasma channels formed to face the plurality of electrodes via the liquid crystal layer and the dielectric layer, the plasma channels running in a second direction different from the first direction. A plurality of pixel regions are formed in respective crossings of the plurality of electrodes and the plurality of plasma channels. Portions of the liquid crystal layer included in the plurality of pixel regions change their orientation states depending on a voltage applied between the electrodes and the plasma channels, to realize display with light having passed the plurality of pixel regions. The liquid crystal display device further includes dielectric structures formed between the electrodes and the liquid crystal layer in periphery zones that include sides of the plurality of pixel regions orthogonal to the second direction. A voltage applied to each of the portions of the liquid crystal layer included in the plurality of pixel regions is smaller in the periphery zone than in the other portion of the pixel region.
The dielectric structures are preferably formed of a transparent polymer material.
Each of the dielectric structures is preferably formed to cover a gap between two adjacent electrodes among the plurality of stripe electrodes and sides of the two electrodes facing each other.
The dielectric structures are preferably formed so that in the periphery zone, the thickness of each of the portions of the liquid crystal layer included in the plurality of pixel regions is nine-tenths or less of the thickness in the other portion of the pixel region.
The dielectric structures are preferably formed so that in the periphery zone, the thickness of each of the portions of the liquid crystal layer included in the plurality of pixel regions is two-thirds or more of the thickness in the other portion of the pixel region.
The device may further include a black matrix formed on the substrate between the plurality of electrodes.
The dielectric structures may be formed of a material having a relative dielectric constant greater than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The device may further includes a high dielectric layer formed between the plurality of electrodes including spaces between the plurality of electrodes and the liquid crystal layer, and the high dielectric layer may be formed of a material having a relative dielectric constant greater than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The dielectric structures may be formed of a material having a relative dielectric constant smaller than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The liquid crystal layer may include a liquid crystal material having negative dielectric anisotropy.
According to the first aspect of the present invention, there is provided a PALCD device capable of suppressing/preventing degradation in display quality due to a crosstalk phenomenon.
In the PALCD device of the present invention, with the existence of the dielectric structures in the periphery zones of the pixel regions, the voltage applied to the periphery zones is smaller than that applied to the other portions of the pixel regions. This suppresses/prevents a crosstalk phenomenon. If the dielectric structures are made of a transparent material, light passing through the periphery zones are usable for display. Decrease in aperture ratio is therefore prevented.
Disorder in orientation of liquid crystal molecules due to a lateral electric field can be prevented by setting the relative dielectric constant of the dielectric structures at a value larger than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material. This suppresses/prevents an SSC-induced crosstalk phenomenon further effectively.
Disorder in orientation of liquid crystal molecules due to a leak electric field can be prevented even when the relative dielectric constant of the dielectric structures is smaller than the relative dielectric constants of the liquid crystal material (larger one of xcex5// and xcex5xe2x8axa5, more preferably, the absolute of relative dielectric constant anisotropy (xcex94xcex5)). This suppresses/prevents a crosstalk phenomenon further effectively.
The crosstalk suppressing effect of the present invention is prominent in normally black mode PALCD devices. Among the devices, the effect is especially prominent in a PALCD device using a liquid crystal material having negative dielectric anisotropy.
The present invention is applicable to display devices other than the plasma addressed liquid crystal display devices, where a crosstalk phenomenon occurs in substantially the same mechanism as DDC and SSC described above.
The liquid crystal display device of the second aspect of the present invention includes: a substrate; a dielectric layer; a liquid crystal layer interposed between the substrate and the dielectric layer; a plurality of stripe electrodes formed on a surface of the substrate facing the liquid crystal layer, the electrodes running in a first direction; and a plurality of stripe plasma channels formed to face the plurality of electrodes via the liquid crystal layer and the dielectric layer, the plasma channels running in a second direction different from the first direction. A plurality of pixel regions are formed in respective crossings of the plurality of electrodes and the plurality of plasma channels, and the device further includes a plurality of first dielectric structures running in the first direction and a plurality of second dielectric structures running in the second direction, formed on the surface of the substrate facing the liquid crystal layer. The liquid crystal layer is divided into a plurality of liquid crystal regions by the plurality of first and second dielectric structures,. Liquid crystal molecules in the plurality of liquid crystal regions are aligned axially symmetrically with respect to an axis vertical to the surface of the substrate. Each of the plurality of pixel regions includes at least one of the plurality of liquid crystal regions. Portions of the liquid crystal layer included in the plurality of pixel regions change their orientation states depending on a voltage applied between the electrodes and the plasma channels, to realize display with light having passed the plurality of pixel regions. In this liquid crystal display device, part of the plurality of first dielectric structures are formed in periphery zones, the periphery zones including sides of the plurality of pixel regions orthogonal to the second direction. A voltage applied to each of the portions of the liquid crystal layer included in the plurality of pixel regions is smaller in the periphery zone than in the other portion of the pixel region.
The first and second dielectric structures are preferably formed of a transparent polymer material.
Each of the first dielectric structures formed in the periphery zones is preferably formed to cover a gap between two adjacent electrodes among the plurality of stripe electrodes and sides of the two electrodes facing each other.
Each of the plurality of pixel regions preferably includes at least two liquid crystal regions adjacent in the second direction, and the width of the first dielectric structure formed between the at least two liquid crystal regions among the plurality of first dielectric structures is preferably smaller than the width of the first dielectric structure formed in the periphery zone. In the case where each of the plurality of pixel regions includes at least two liquid crystal regions adjacent in the first direction, the width of the second dielectric structure formed between the at least two liquid crystal regions is preferably smaller than the width of the first dielectric structure formed in the periphery zone. Each of the plurality of pixel regions may include one liquid crystal region.
The first dielectric structures formed in the periphery zones are preferably formed so that in the periphery zones, the thickness of each of the portions of the liquid crystal layer included in the plurality of pixel regions is nine-tenths or less of the thickness in the other portion of the pixel region.
The first dielectric structures formed in the periphery zones are preferably formed so that in the periphery zone, the thickness of each of the portions of the liquid crystal layer included in the plurality of pixel regions is two-thirds or more of the thickness in the other portion of the pixel region.
The device may further include a black matrix formed on the substrate between the plurality of electrodes.
The width of the first dielectric structure located between the two electrodes is preferably larger than the width of the first dielectric structures located on the two electrodes, and liquid crystal regions where liquid crystal molecules in the liquid crystal layer are aligned axially symmetrically with respect to an axis vertical to the surface of the substrate are preferably formed below the first dielectric structures.
The third dielectric structures may further be formed on the first dielectric structures, and the liquid crystal molecules in the liquid crystal layer located below the first dielectric structures may be aligned axially symmetrically by the existence of the third dielectric structures.
The device may further include a black matrix formed on the substrate between the plurality of electrodes, and the width of the black matrix may be smaller than the width of the first dielectric structure located between the two electrodes.
The device may further include a black matrix formed on the substrate between the plurality of electrodes, and the plurality of first dielectric structures and the plurality of second dielectric structures may define a plurality of apertures on the black matrix.
The first dielectric structures may be formed of a material having a relative dielectric constant greater than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The device may further include a high dielectric layer formed between the plurality of electrodes including spaces between the plurality of electrodes and the liquid crystal layer, and the high dielectric layer may be formed of a material having a relative dielectric constant greater than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The first dielectric structures may be formed of a material having a relative dielectric constant smaller than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material included in the liquid crystal layer.
The liquid crystal layer may include a liquid crystal material having negative dielectric anisotropy.
According to the second aspect of the present invention, there is provided a PALCD device that can suppress/prevent degradation in display quality due to a crosstalk phenomenon and has high viewing angle characteristics.
In the PALCD device of the present invention, with the existence of the dielectric structures in the periphery zones of the pixel regions, the voltage applied to the periphery zones is smaller than that applied to the other portions of the pixel regions. This suppresses/prevents a crosstalk phenomenon. If the dielectric structures are made of a transparent material, light passing through the periphery zones are usable for display. Decrease in aperture ratio is therefore prevented. Such dielectric structures can be formed of the same material as that used for other dielectric structures for dividing the liquid crystal layer. Increase in fabrication step is therefore prevented.
Disorder in orientation of liquid crystal molecules due to a lateral electric field can be prevented by setting the relative dielectric constant of the dielectric structures at a value larger than the absolute of relative dielectric constant anisotropy (xcex94xcex5) of a liquid crystal material. This suppresses/prevents an SSC-induced crosstalk phenomenon further effectively.
Disorder in orientation of liquid crystal molecules due to a leak electric field can be prevented even when the relative dielectric constant of the dielectric structures is smaller than the relative dielectric constants of the liquid crystal material (larger one of xcex5// and xcex5xe2x8axa5, more preferably, the absolute of relative dielectric constant anisotropy (xcex94xcex5)). This suppresses/prevents a crosstalk phenomenon further effectively.
The crosstalk suppressing effect of the present invention is prominent in normally black mode PALCD devices. Among the devices, the effect is especially prominent in a PALCD device using a liquid crystal material having negative dielectric anisotropy.
As described above, the present inventors examined in detail the causes of the crosstalk phenomenon occurring in PALCD devices. The present invention was only attained from the knowledge obtained from the examination. To state more specifically, the present invention is based on the idea that a major cause of the crosstalk phenomenon is a leak electric field (voltage) to adjacent pixel regions due to DDC and/or SSC, and that in order to suppress/prevent the crosstalk phenomenon, the magnitude of such a leak electric field (voltage) applied to the liquid crystal layer in the adjacent pixel regions should be sufficiently reduced.
The crosstalk phenomenon occurs between adjacent pixel regions along the length of the plasma channel, that is, between adjacent pixel regions including adjacent different electrodes. In view of this, the crosstalk phenomenon can be suppressed/prevented in the following manner. That is, the voltage applied to portions of each pixel region including its sides orthogonal to the length of the plasma channel (sides parallel to the length of the stripe electrode) (hereinafter, such a portion is called a xe2x80x9cperiphery zonexe2x80x9d) is made smaller than the voltage applied to the other portion of the pixel region (other area). For this purpose, a dielectric structure is formed in the periphery zone.
For convenience of the description on the crosstalk phenomenon occurring between adjacent pixel regions along the length of the plasma channel, the following terms are defined as follows unless otherwise specified. The sides among all the sides of the electrodes, the dielectric structures, and the pixel regions that are orthogonal to the length of the plasma channel are called xe2x80x9cperiphery sidesxe2x80x9d. The electrodes and the dielectric structures, each of which is typically rectangular, are arranged in a stripe pattern. The first and second dielectric structures for axially symmetrical alignment (these structures may also be called xe2x80x9cdielectric wallsxe2x80x9d) are formed to cross each other, forming a lattice as a whole. The first and second dielectric structures are wall-like structures running in parallel with the length of the electrodes and the length of the plasma channels, respectively. The pixel regions, each of which is rectangular, are arranged in a matrix. The distance between a pair of periphery sides is called a xe2x80x9cwidthxe2x80x9d. The portion of each pixel region that includes a periphery side is called a xe2x80x9cperiphery zonexe2x80x9d. Typically, at least part of the dielectric structure is selectively formed in the periphery zone. The width of each of the dielectric structures (including the first and second dielectric structures) refers to the width thereof orthogonal to the length of the dielectric structure.
The PALCD device of the present invention includes a dielectric structure formed between the electrode and the liquid crystal layer in each periphery zone for reducing the voltage applied to the periphery zone compared with the voltage applied to the other area. Therefore, the thickness of the liquid crystal layer in the periphery zone including the dielectric structure is smaller than that in the other area. As a result, the voltage applied to the liquid crystal layer in the periphery zone is smaller than the voltage applied to the liquid crystal layer in the other area. By this reduction in the thickness of the liquid crystal layer in the periphery zone as well as the reduction in the voltage applied to the liquid crystal layer in the periphery zone, the crosstalk phenomenon can be effectively suppressed/prevented. This is due to the following two factors.
The first factor will be described with reference to FIG. 26. FIG. 26 is a graph showing typical voltage-transmittance curves L1, L2, and L3 of LCD devices having liquid crystal layers different in thickness. As shown in FIG. 26, in general, as the thickness of the liquid crystal layer is smaller (d1 greater than d2 greater than d3), the transmittance of the LCD device decreases (T1 greater than T2 greater than T3) when the same voltage is applied (directly to the liquid crystal layer). This is the first factor. Further, as the second factor, in the case of a PALCD device, as the thickness of the liquid crystal layer is smaller, the voltage applied to the liquid crystal layer decreases when the same voltage is applied (to the dielectric sheet and the liquid crystal layer). Due to the above two factors, as is apparent from the voltage-transmittance curves L1xe2x80x2, L2xe2x80x2, and L3xe2x80x2 shown in FIG. 27, as the thickness of the liquid crystal layer is smaller (d1xe2x80x2 greater than d2xe2x80x2 greater than d3xe2x80x2), the transmittance of the LCD device decreases (T1xe2x80x2 greater than T2xe2x80x2 greater than T3xe2x80x2). The x-axis of the graph in FIG. 27 represents the voltage applied to the liquid crystal layer.
Thus, in the PALCD device where the second factor described above functions, the crosstalk phenomenon can be effectively suppressed/prevented by providing the dielectric structures in the periphery zones. More specifically, the voltage applied between the electrode and the dielectric bottom sheet is divided according to capacitances formed by the dielectric bottom surface (virtual electrode)/dielectric sheet/liquid crystal layer/dielectric structure/electrode. Assuming that the relative dielectric constant of the dielectric structure is equal to those of the liquid crystal layer and the dielectric sheet, the voltage applied to the liquid crystal layer in the periphery zone is divided in proportion to the thicknesses of the dielectric sheet, the liquid crystal layer, and the dielectric structure (see expression (2) above). Accordingly, by adjusting the thickness of the dielectric structure, the thickness of the liquid crystal layer in the periphery zone is controlled and thus the magnitude of the voltage applied to the liquid crystal layer in the periphery zone is adjusted. By this adjustment, the crosstalk phenomenon can be suppressed/prevented. Strictly, the voltage is divided according to the capacitances formed between the electrode and the dielectric bottom surface as described above. Therefore, the thickness of the dielectric structure should be determined in consideration of the relative dielectric constants of the respective components.
The voltage applied to the periphery zone is reduced by forming the dielectric structure, as described above. This means that the threshold voltage (voltage required to change the transmittance) of the liquid crystal layer in the periphery zone is apparently increased compared with that in the other area. Thus, by adjusting the thickness of the dielectric structure to adjust the apparent threshold voltage of the liquid crystal layer in the periphery zone, the crosstalk phenomenon is suppressed/prevented.
If the dielectric structure formed in the periphery zone is made of a transparent polymer material, light passing through the periphery zone can be utilized for display. Therefore, the aperture ratio is prevented from decreasing.
In the construction having two or more liquid crystal regions in one pixel region, a dielectric structure (first and/or second dielectric structure) running across the pixel region is preferably made of a transparent polymer material in view of the aperture ratio. By using the same transparent polymer material for both the dielectric structures formed in the periphery zones and the dielectric structures running across the pixel regions, all the dielectric structures can be formed in one process. If the width of the dielectric structures running across the pixel regions is made smaller than that of the dielectric structures formed in the periphery zones, uniformity of display characteristics in each pixel region is enhanced.
The dielectric structure may be formed so that at least part thereof serves to reduce a leak electric field (voltage) from the adjacent pixel region. The dielectric structure may be formed at least in the periphery zone of the pixel region, so that the effect of suppressing the crosstalk phenomenon is obtained. In other words, the width of the pixel region in display can be changed by controlling the position and the thickness of the dielectric structure. Therefore, the dielectric structure may be formed so that it is located in the periphery zone of the resultantly-obtained pixel region in display (that is, so that the periphery of the pixel region in display is located within the width of the dielectric structure, or so that the dielectric structure is located near the periphery side of the pixel region in display inside the pixel region). In this way, the crosstalk phenomenon can be suppressed. The relationship between the position of the dielectric structure and the pixel region in display will be described in detail in the preferred embodiments of the present invention.
Typically, the dielectric structure may be formed so as to cover the gap between the two adjacent stripe electrodes and the sides of the electrodes facing each other. This simplifies the structure and the fabrication process.
In order to sufficiently lower the leak electric field (voltage) that is a cause of the crosstalk phenomenon, the dielectric structure is preferably formed so that the thickness of the liquid crystal layer in the periphery zone is nine-tenths or less of the thickness of the liquid crystal layer in the other area. In other words, the thickness of the dielectric structure is preferably one-tenth or more of the thickness of the liquid crystal layer in the portion of the pixel region having no dielectric structure. Also, the thickness of the dielectric structure is preferably one-third or less of the thickness of the liquid crystal layer in the portion of the pixel region having no dielectric structure. If the dielectric structure is thicker than the above value, the voltage decreases excessively, failing to apply a sufficient voltage to the portion of the liquid crystal layer located below the dielectric structure. As a result, display brightness or the aperture ratio may be reduced. Moreover, if the dielectric structure itself has a low transmittance for visible light, this is observed as reduction in display brightness. The transmittance of the dielectric structure is therefore preferably 95% or more. If the thickness of the dielectric structure is 2 xcexcm or less, transmittance of 95% or more can be attained using any of a variety of types of transparent polymer materials. It should be noted, however, that the orientation state of the portions of the liquid crystal layer located below the dielectric structures is not necessarily the same as that of the portions of the liquid crystal layer in the areas sandwiched by the dielectric structures. Therefore, the display brightness may be varied with the difference in the orientation state of the liquid crystal layer. As a result, there is a case that difference in display brightness is not visually recognized even if the transmittance of the dielectric structures is below 95%. Accordingly, the transmittance of the dielectric structures themselves may be determined appropriately as long as no difference in display brightness is visually recognized between the portions of the pixel region having the dielectric structures and the portions thereof having no dielectric structures.
In addition, SSC can be effectively suppressed/prevented in the following manner. The relative dielectric constant of the dielectric structures formed to cover at least the sides of the electrodes and/or the spaces between the electrodes is set at a value larger than the absolute of the relative dielectric constant anisotropy (xcex94xcex5) of the liquid crystal material. By this setting, it is possible to induce a larger number of electric lines of power generated between the adjacent electrodes into the dielectric structures than into the liquid crystal layer. That is, it is possible to selectively induce lateral electric lines of power generated between the adjacent electrodes having a potential difference into the dielectric structures, so as to reduce the number of electric lines of power (the intensity of the electric field) generated in the liquid crystal layer. The relative dielectric constant of the dielectric structures is preferably larger than the larger one of the relative dielectric constants (xcex94// and xcex5xe2x8axa5) of the liquid crystal material. Instead of increasing the relative dielectric constant of the dielectric structures, it is possible to additionally form a high dielectric layer (layer having a relative dielectric constant larger than the absolute of the relative dielectric constant the anisotropy (xcex94xcex5) of the liquid crystal material). The relative dielectric constant of the high-dielectric layer is preferably larger than the larger one of the relative dielectric constants (xcex5// and xcex5xe2x8axa5) of the liquid crystal material. It is naturally possible to combine the dielectric structures having a high relative dielectric constant with the high dielectric layer. The high dielectric layer may be formed on the top or bottom of the dielectric structures.
Alternatively, the crosstalk phenomenon due to a leak electric field (including SSC) can be effectively suppressed even with the dielectric structures made of a material having a relative dielectric constant smaller than the absolute of the relative dielectric constant the anisotropy (xcex94xcex5) of the liquid crystal material. Electric lines of power output from the portion of the electrode located in the periphery zone where the dielectric structure is formed are weakened during the passing of the lines through the dielectric structure. As a result, the intensity of the electric lines of power generated between the adjacent electrodes (pixel regions) is lowered, thereby weakening the electric field (voltage) applied to the portion of the liquid crystal layer located below the dielectric structure. The above effect is presumably obtained if the relative dielectric constant of the dielectric structure formed in the periphery zone is smaller than the larger one of the relative dielectric constants (xcex5// and xcex5xe2x8axa5) of the liquid crystal material. It is however preferable to have a relative dielectric constant smaller than the absolute of the relative dielectric constant the anisotropy (xcex94xcex5) of the liquid crystal material.
The SSC-induced crosstalk phenomenon greatly reduces display quality of the PALCD device using a liquid crystal material having negative dielectric anisotropy. The present invention is therefore significantly effective for this type of device.
The crosstalk phenomenon depends on the basic structure of the PALCD device (the widths of the electrodes and the plasma channels, the distance between the electrodes, presence or absence of a black matrix, the drive voltage, and the like). Accordingly, the position and the size of the dielectric structures may be appropriately set depending on the structure of the PALCD device.